This invention concerns a chair with a seat panel that is carried by a central column or several legs, and with a backrest carrier that runs backwards under the seat panel and upwards behind the seat panel and carries a backrest. Near its front edge, in order to provide a tilting movement, the seat panel rests on a horizontal, crosswise running first pivot and, near its back edge, to provide a tilting movement, it is attached to the backrest carrier around a second, parallel pivot. The backrest carrier is attached to the central column or the legs around a third pivot that runs between the first and second pivots and is parallel to them. The springs are arranged so that they exert an upward force on the seat panel and a forward force on the backrest.
A chair of this kind is known from DE 43 13 301 C2. This chair is characterized by the fact that, on the underside of the seat panel, at the front and at the back, downward directed supports are rigidly fastened in pairs. The bottom ends of the front supports are fastened to the bottom ends of downward connecting rods that are fastened to the seat panel carrier to allow for a tilting movement. The bottom ends of the rear supports are connected to the backrest carrier to allow for a tilting movement. The springs are arranged in the form of a coil spring between the seat panel and the seat panel carrier.
With this construction of the chair, there is a so-called synchronous mechanism whereby, if the user leans back on the chair, thereby putting a load on the rear part of the seat panel, the seat panel sinks down and, at the same time, the backrest tilts backwards and this tilting movement works against the force of the springs. As this happens, the angle of tilt of the backrest is normally larger than the angle of tilt of the seat panel. The relationship of the angles of tilt towards each other is determined by the jointly acting lengths of the levers.
The disadvantage of this chair is that the preload force of the backrest depends solely on the strength of the arrangement of the compression springs and on the leverage determined by the construction of the chair. That is why, in this case, it is impossible to adapt the preload force of the backrest to the different body weights of different users of the chair. It would only be possible to influence the preload force of the backrest through an arrangement of springs that can be adjusted or pre-set. However, to do this, the user of the chair would have to carry out the adjustment manually, which would be an undesirably high expenditure of effort, especially if the chair is used by different users of different weights. What is more, it is not guaranteed that the user will actually make the correct adjustment. This could give rise to incorrect adjustments of the preload force of the backrest which, in some cases, could be damaging to the user""s health.
The present invention therefore sets out to create a chair of the kind described above, which avoids the disadvantages that have been illustrated and in which the automatic adjustment of the preload force of the backrest to users of varying weights is possible and in which the constructional outlay, especially the number of individual components that are required, is kept low.
The distinctive features of the chair according to the invention also have the synchronous mechanism whereby, when sat upon, the seat panel moves downwards against the force of the spring arrangement according to the weight of the user. This necessarily leads to a corresponding increase in the stress of the spring arrangement. As the spring arrangement rests on the backrest carrier, the increased stress of the spring arrangement exerts an increased lever moment on the backrest carrier, which gives rise to an increase in the preload force of the backrest. A person using the chair with a heavier bodyweight therefore experiences greater support for his or her back from the backrest, which is desirable and makes ergonomic sense. If a user with a lighter bodyweight sits on the chair, the seat panel takes up a higher position in which the spring arrangement has less stress placed upon it; this necessarily gives rise to a correspondingly lower preload force being exerted on the backrest, so that a user with a lighter bodyweight experiences a commensurately lower support force exerted by the backrest on his or her back, in line with his or her lighter bodyweight. At the same time, the synchronous adjustment of the seat panel and the backrest is completely preserved, so that, if the position of the seat panel is changed, the seat panel and the backrest tilt in a fixed relationship to each other.
Despite the synchronous mechanism and the automatic adjustment of the preload force exerted on the backrest to the different bodyweights of the users, the chair is of a surprisingly simple construction, so that, in relation to the functions that are offered, the manufacture of the chair is simple and cost-effective. In particular, a separate seat panel carrier is no longer required since the spring arrangement carries the seat panel close to its front edge and positions the seat panel so that it can tilt. The spring arrangement can easily be bent into the necessary shape. It is possible to modify the force of the springs without any problem by changing the strength of the material of the spring-steel rod or wire or by changing the lengths of the legs of the spring.
In order to achieve an especially flat design of the mechanism which requires less height under the seat panel, it is preferable that the backrest carrier has an extension going forward beyond the third pivot to form a lever arm and that the spring rests on this backrest carrier extension on which it exerts a downward force. This design of the mechanism that is necessary for the movement of the chair also gives the chair an attractive appearance and is especially suitable if the chairs are to be stacked.
The chair has also been designed so that the spring arrangement is formed by two springs which, seen from above, are bent into the shape of a U, and are arranged next to each other under the seat panel as a mirror image. The first U-shaped leg of the spring is clamped to the chair column or chair legs and points backwards, the U-shaped bend of the spring forms the section of the spring attached to the seat panel for the first pivot and the second U-shaped leg of the spring rests on the extension of the backrest carrier. These U-shaped springs are particularly flat in construction and are easy to manufacture as bending components. The use of two springs ensures that the forces are well distributed and avoids undesired lateral tipping movements of the seat panel.
A development of the design that has been illustrated above consists in combining the two U-shaped bent springs that form the spring arrangement into a one-piece spring that is shaped, seen from above, into a W, and is arranged underneath the seat panel symmetrically to the longitudinal central axis of the seat panel. This reduces production and assembly costs when the chair is being manufactured.
It is also proposed that a clamping device, preferably a gripping sleeve, is firmly attached to the chair column or chair legs for each of the first legs of the spring. In this way, arranging the springs is easy and, at the same time, the resulting arrangement is visually very unobtrusive.
An alternative design of the chair allows for the backrest carrier to terminate at the third pivot, and for the springs behind the third pivot to rest on the backrest carrier and to exert an upward force on the backrest carrier. This design also enables the functions illustrated above to be carried out. In this case, a different spring arrangement can be used in view of its working direction. This increases technical freedom during the manufacture of the chair and, in particular, in the choice of the springs that are to be used.
A development of the design of the chair described above is characterized by the fact that the spring arrangement is formed by means of two tangentially loaded helical springs, each with a coiled spring body and two legs.
In this case, the springs are arranged next to each other under the seat panel in a mirror imagexe2x80x94the spring body surrounds the third pivot, the first leg of the spring, pointing forwards, rests against the seat panel and its front end forms the spring section for the first pivot, which is attached to the seat panel, and the second leg of the spring, pointing backwards, rests on the backrest carrier. It is true that tangentially loaded helical springs need more height but their spring features are easier to influence and determine. What is more, they generally allow for greater spring movement and hence greater chair comfort.
In a further, more advantageous, arrangement, the two tangentially loaded helical springs forming the spring arrangement are combined into a one-piece double tangentially loaded helical spring, which is arranged under the seat panel symmetrically to the longitudinal central axis of the seat panel. This simplifies the manufacture and assembly of the chair mechanism.
As regards the design of the chair using tangentially loaded helical springs, it is preferable that a stop, preferably a stop plate, is attached to the chair column or the chair legs for each of the first legs of the spring, in order to restrict its upward movement. In this way, a defined upper end position for the seat panel is established when the chair is not in use. This stops the seat panel from rising excessively high when there is no load on it.
Instead of the spring arrangement described above, other, alternative, spring arrangements can be used, provided the necessary forces are exerted in the required directions as illustrated above.
Preferably, in all the chair designs at least two front brackets and at least two rear brackets are attached to the underside of the seat panel, through which the first and second pivots run respectively. In this way, the forces that are exerted when the chair is in use are conveniently directed towards, and lead away from, the seat panel. What is more, it is easy to attach a modified seat panel which, together with an equally easily replaced attached backrest, allows the chair to be adjusted in a myriad of ways.
The invention also allows a bearing pin in the shape of a rod or tube to be placed beneath the seat panel to form the third pivot. This is firmly attached to the chair column or chair legs and the backrest carrier rests on it in such a way that it can tilt. Apart from forming the third pivot, this pin also serves as a cross brace, which increases the stability of the chair, especially when it is designed with four legs.
As explained above, the chair preferably has a combination of the synchronous mechanism and weight-dependent backrest preload. For users who do not want the synchronous mechanism, the chair can alternatively be designed so that the synchronization of the seat panel with the backrest carrier and the second rear pivot are omitted, so that the seat panel and the backrest carrier can tilt in an unsynchronized manner, i.e., independently of each other. The necessary technical modifications are restricted to omitting the individual components, in particular the rear carrier pin and the accompanying brackets.